Aerosol optical particle sensors are important in a number of fields. For example, the micro-contamination industry is reliant on the use of aerosol optical particle sensors to provide a quantitative assessment of contamination. Aerosol optical particle sensors are employed for point of use measurement of air-born particle contamination in clean-rooms and clean zones. Generally, these particle sensors are low cost and do not rely on internal pumps to generate the required airflow through the sensor. The particle sensors instead rely on connection to house vacuum systems in order to generate flow. The flow rate is generally controlled by connecting the vacuum system to a low cost critical flow orifice located in the particle sensor. In order to accurately quantify the concentration of particles detected by the particle sensor, the flow-rate of the gas introduced to the sensor must be known. For example, an undetected flow-rate decrease of 15% would lead to a particle contamination concentration level determination that is 15% less than actual. Accordingly, it is important that the flow-rate of the gas being sampled by the sensor be either measured or accurately alarmed to provide the user with an indication that the flow-rate has deviated from a certain user-specified level.
A critical flow orifice to control volumetric flow rate is a well known technique to help ensure flow-rate is maintained. Critical flow orifices are well established in the art (see, e.g., Willeke/Baron, “Aerosol Measurement”; and Hinds, “Aerosol Technology”). Although a critical flow orifice is a very good low cost volumetric flow control device, such an orifice does not provide the capacity to monitor volumetric flow rate in a low cost and accurate manner. Off-the-shelf flow sensors can be utilized, but cost hundreds of dollars per particle sensor. Internal flow monitoring can be added to the particle sensor, such as described in U.S. Pat. No. 6,167,107, Air Pump for Particle Sensing Using Regenerative Fan, and Associated Methods, Jul. 16, 1999. Such internal flow-monitoring, however, also requires flow sensing devices that in total cost greater than one hundred dollars. Such costs make the use of such devices in low cost aerosol optical particle sensors impractical.
Accordingly, there is a need for an intelligent flow monitoring system that can accurately and reliably monitor flow rate in a particle sensor without significantly adding to the cost or complexity of the particle sensor. One object of the invention, therefore, is to provide devices and methods for monitoring flow rate in a particle sensor and identifying when the flow rate has deviated from a target flow rate so that appropriate remedial action may be taken.